Gas sensor apparatuses that measure environmental characteristics, such as concentration and temperatures of a gas, are used in various technical fields. In the internal combustion engine for a vehicle, for example, an intake air amount and an intake air temperature are measured to optimize the fuel injection amount in order to reduce fuel consumption. For the optimal operation of the internal combustion engine, high accuracy measurement of environmental parameters of the concentration, such as temperatures and humidity, (which will simply be referred to as the concentration hereinafter) is required. The environmental sensor elements that measure temperatures and concentration are also used in the internal combustion engine for a hydrogen fuel.
When the environmental sensor elements are installed in an air intake passage of the internal combustion engine, air (gas) that flows through a main passage is partially taken into a sub-passage of a housing, and the concentration, such as humidity, is measured by the concentration sensor element stored in a measurement chamber that communicates with the sub-passage via a gas intake port. In this structure, the concentration sensor element, which measures the concentration, is not directly exposed to the air flow. As a result of this, a dust-proof effect, or an effect of reducing breakage of the sensor element due to collision of particles, can be obtained. When the concentration sensor element is disposed closer to a pipe wall than to the sub-passage, a temperature of the internal combustion engine increases and the temperature increase causes heat transmission from the internal combustion engine via a pipe wall. The measurement accuracy of the temperature is thus adversely affected. To prevent this, a gas sensor apparatus in which the measurement chamber is disposed closer to the center of the passage away from the passage wall of the main passage instead of the sub-passage is known (e.g., PTL 1).